Prior to ignition (i.e. prior to a state of ionization being established), a fluorescent lamp has a very high impedance, whereas the impedance falls to a much lower value after ignition takes place. Thus, such a lamp has a form of negative-resistance characteristic. For this reason, it is necessary to insert some form of ballast impedance in series with a fluorescent lamp, e.g. a resistor or inductor, in order to ensure stable operation. However due to the change in impedance which takes place upon ignition of the fluorescent lamp, a substantial voltage drop is produced across such a ballast impedance when the lamp is in operation. That is to say, a much higher voltage must be applied to the combination of fluorescent lamp and ballast than is actually required to operate the lamp after ignition has been established. Typically, the applied voltage may be as much as twice the value of voltage which is required to operate the lamp, so that a substantial amount of power is wasted by such a ballast impedance.
In recent years, a method of supplying power to fluorescent lamps has come into use whereby the lamp is driven by a high-frequency drive voltage generated by a semiconductor inverter circuit. The output voltage from the inverter is generally boosted by a step-up transformer, and in this case the necessary ballast impedance may be provided by the leakage inductance of the transformer. However even with such an arrangement, the output voltage from the transformer prior to ignition is generally of the order of twice the voltage actually required to operate the lamp following ignition, so that again a substantial amount of power loss is incurred. As a result of this, it has only been found possible to attain an increase in efficiency of operation of a fluorescent lamp, using such a high-frequency drive voltage method, of the order of 20%, by comparison with conventional drive methods.
There is therefore a requirement for means for supplying power to a fluorescent lamp whereby it will not be necessary to apply a drive voltage, prior to ignition of the lamp, which is considerably greater than the voltage actually required to operate the lamp after ignition is achieved, but whereby ignition will be rapidly and reliably established. Such means are provided by a power supply apparatus for a fluorescent lamp according to the present invention.